18650 On-Board Charging and protecting the circuit

Thread Starter

patrickstefanski

Joined Apr 15, 2017
46
I'm making a project where it will be hard to take the batteries in and out, so I want to build an on-board charging port. I'm thinking of just adding two connectors to the BMS output, One for Charging and one going to the circuit. My concern is if the circuit will get damaged while charging is going on. Do I need any other protection between the BMS and the circuit?

Minor question: For purposes of space, I'm thinking of adding my power switch between the battery and the BMS. Is there any reason why I shouldn't do this?

Batteries: 3x18650 cells.
BMS: Cheap BMS I'm using mainly for low voltage protection, I will be charging with my balance charger (Tenergy).
 

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Dodgydave

Joined Jun 22, 2012
8,678
Most Bms pcbs have the same terminals as Output and Charger, so you don't need to worry about the batteries, your only worry is if the charger output is higher in voltage than your circuit voltage. If so then you need to isolate the circuit when on charge.
 

Thread Starter

patrickstefanski

Joined Apr 15, 2017
46
Most Bms pcbs have the same terminals as Output and Charger, so you don't need to worry about the batteries, your only worry is if the charger output is higher in voltage than your circuit voltage. If so then you need to isolate the circuit when on charge.
Cool, yes, on this BMS board the input/output is on the same pads. This may be a dumb question, but the charger output wouldn't be higher than the voltage of the battery, would it? I'm charging 3S, what kind of charger output could I expect?
 

jpanhalt

Joined Jan 18, 2008
8,487
I have used a switching jack for doing charging and on/off functions with one relatively small device. With the charge plug plug inserted, the battery is isolated from the circuit, so there is no risk from a higher charging voltage. With the plug removed, the circuit is on. With a blank plug inserted, the circuit is off. Here's an example:

upload_2018-12-27_16-7-11.png

Note, the battery negative, which is connected to the "ring," gets switched.
 

Dodgydave

Joined Jun 22, 2012
8,678
You need to isolate your circuit not the battery, the charger usually outputs 4.2V per cell, so 3S charger will give out 12.6V...
 

jpanhalt

Joined Jan 18, 2008
8,487
The schematic I posted was missing a label. It was for my personal use and the board is correct. Here is the missing label:

upload_2018-12-28_4-12-21.png

The circuit is isolated during charging, as there is no return path to the battery/negative supply of the charger, i.e., no common.
 
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